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The eggs are excreted
unembryonated in the sputum, or alternately they are swallowed and passed
with stool
. In the
external environment, the eggs become embryonated
, and
miracidia hatch and seek the first intermediate host, a snail, and penetrate
its soft tissues
.
Miracidia go through several developmental stages inside the snail
:
sporocysts
, rediae
, with
the latter giving rise to many cercariae
, which
emerge from the snail. The cercariae invade the second intermediate host, a
crustacean such as a crab or crayfish, where they encyst and become
metacercariae. This is the infective stage for the mammalian host
. Human
infection with P. westermani occurs by eating inadequately cooked or
pickled crab or crayfish that harbor metacercariae of the parasite
. The
metacercariae excyst in the duodenum
,
penetrate through the intestinal wall into the peritoneal cavity, then
through the abdominal wall and diaphragm into the lungs, where they become
encapsulated and develop into adults
(7.5 to 12
mm by 4 to 6 mm). The worms can also reach other organs and tissues, such
as the brain and striated muscles, respectively. However, when this takes
place completion of the life cycles is not achieved, because the eggs laid
cannot exit these sites. Time from infection to oviposition is 65 to 90
days. Infections may persist for 20 years in humans. Animals such as pigs, dogs, and a variety of feline species can also harbor P. westermani.
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